The long-term reliability and safety of implantable cardiac leads is a significant issue. Anomalies of conductors in implantable medical devices constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). For example, early diagnosis of ICD lead conductor anomalies is important to reduce morbidity and/or mortality from loss of pacing, inappropriate ICD shocks, and/or ineffective treatment of ventricular tachycardia or fibrillation (ventricular fibrillation). The early diagnosis of conductor anomalies for implantable cardiac leads is a critically important step in reducing these issues and making ICDs safer.
Multilumen ICD defibrillation electrodes or leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneous or intravascular leads. Insulation failures have been known to result in a functional failure of the corresponding conductor. Functional failure of a pace-sense conductor may result in symptoms caused by loss of pacing functions for bradycardia, cardiac resynchronization, or antitachycardia pacing. Functional failure of a high-voltage conductor may result in fatal failure of cardioversion or defibrillation.
Thus, one major goal is high sensitivity of diagnosis: identification of lead insulation failures at the subclinical stage, before they present as a clinical problem. A second major goal is high specificity: a false positive provisional clinical diagnosis of lead insulation failure may trigger patient anxiety and lead to potentially avoidable diagnostic testing. A false positive clinical diagnosis of insulation failure results in unnecessary lead replacement, with corresponding expense and surgical risk. Any clinical method for detecting conductor anomalies in implanted leads must make measurements while the conductor and lead are in the body.
In addition to limited sensitivity, present methods for diagnosing lead conductor anomalies have limited specificity resulting in false positive diagnostics. Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks. In the only report on this subject, 23% of leads extracted for the clinical diagnosis of lead fracture tested normally after explant.
Insulation failures occur most commonly at three regions along the course of a pacemaker or ICD lead. The first region is within the pocket, caused either by abrasion of the lead insulation by pressure from the housing (“CAN”) of the pulse generator or twisting of the lead within the pocket. The second region is that between the clavicle and first rib, where the lead is subject to “clavicular crush.” The third region is the intracardiac region between or under the shock coils. This third region is a particularly common site of insulation failure for the St. Jude Riata® lead which is subject to “inside-out” insulation failure due to motion of the internal cables relative to the outer insulation. In this case, inside-out abrasion of the cable to the right-ventricular shock coil may abrade against the proximal (superior vena cava) shock coil, resulting in a short circuit within the lead.
Most commonly, insulation failures of ICD defibrillation leads within the pocket can result in abrasion of the insulation around the conductor of the right-ventricular defibrillation coil (coil-CAN abrasion). This abrasion results in a short circuit between the CAN electrode and the right ventricular defibrillation coil. This short circuit prevents defibrillation current from reaching the heart in the event of life threatening ventricular tachycardia or fibrillation. In the case where the shock is delivered, extremely high current flowing through the shorted output circuit of the ICD may irrevocably damage the generator's components. Thus, many modern ICDs contain circuits to protect the ICD against shorted high voltage outputs by aborting the shock if the current in the output circuit is sufficiently high during a shock. However, even though such protective circuitry prevents damage to the generator, it also detrimentally withholds potentially lifesaving therapy from the patient.
Existing technology for diagnosis of conductor anomalies in an ICD lead is believed to have significant limitations and shortcomings. What is desired is a method that could analyze and identify implantable cardiac lead conductor anomalies at the subclinical stage, before they present as a clinical problem, and do so with a high sensitivity and specificity that minimizes false positives for implantable cardiac lead conductor anomalies. In particular, a method for timely and accurate diagnosis of insulation failures of ICD defibrillation leads within the pocket that results in a short circuit between the CAN electrode and the right-ventricular defibrillation coil is needed.